How the numbers are calculated
Everything starts from the total rise: the vertical distance from the finished floor at the bottom to the finished floor at the top. The calculator then works out:
risers = round( total rise ÷ target riser height )
riser height = total rise ÷ risers
treads = risers − 1 (or = risers if you build a full tread at the top)
total run = treads × tread depth
pitch angle = arctan( riser height ÷ tread depth )
stringer length = √( total rise² + total run² )
The pitch angle is the slope of the line through the stair nosings, the angle you set on a framing square. The stringer length is the classic diagonal of the rise and run triangle.
Reading the plans
The Plans tab holds three sheets that print as separate pages: the side profile with rise, run and pitch angle, the stringer layout, and the tread detail.
Tread detail
The tread sheet shows a top view of one board and a section through two steps, plus a cut list. The board is as wide as the stair and as deep as the going plus the nosing, the part that projects past the riser. With solid risers and treads under 11 in, the IRC asks for a nosing between ¾ and 1¼ in; the calculator checks it. With open risers it also checks that a 4⅜ in sphere cannot pass through the gap on stairs more than 30 in above the ground.
Stringer layout
The stringer sheet draws one board laid flat, with the notches cut along its top edge, the way it is marked out before cutting. Two settings shape it:
- Stringer stock: 2×10 (9¼ in), 2×12 (11¼ in) or 2×14 (13¼ in) actual depth. Deeper stock leaves more wood under the notches and needs a shorter board.
- Tread thickness: the stringer sits one tread thickness below the finished steps, so the first riser cut is shorter than the others by that amount. Without this, the bottom step comes out too tall once the tread is on.
The result reports the minimum board length, measured along the underside, and the throat: the wood left under the deepest notch. Many builders want at least 3½ in there, and the calculator flags anything below that.
How many stringers you need depends on the stair width and on what the treads are made of, and the IRC does not set a spacing for them. The calculator does not decide that for you: the layout is for one board, and you cut as many as your design calls for.
Limits used for the code check
The check uses the International Residential Code (IRC) as adopted for one- and two-family dwellings:
- Riser height: 7¾ in (7.75 in) maximum.
- Tread depth: 10 in minimum.
- Stairway width: 36 in minimum.
- Vertical rise of one flight: 12 ft 7 in (151 in) maximum between floors or landings.
Comfort is separate from code. A common rule of thumb is that two risers plus one tread should add up to about 24 to 25 inches. The calculator shows that as a note, not as a pass or fail.
This tool gives estimates for planning. Local codes differ and are updated often, so confirm the requirements with your building department and verify every dimension on site before cutting.
Frequently asked questions
What is the maximum riser height allowed by the IRC?
The International Residential Code limits riser height to 7¾ inches (7.75 in). Treads must be at least 10 inches deep. Local codes can be stricter, so confirm with your building department.
How do I find the number of risers for my staircase?
Divide the total rise (finished floor to finished floor) by the riser height you would like, then round to the nearest whole number. Divide the total rise by that whole number to get the exact height of every riser.
Why is there one fewer tread than risers?
When the stair ends flush with the upper floor, that floor acts as the last tread, so a straight stair with 14 risers has 13 treads. If you build a full tread at the top instead, the calculator can count it: turn off "Top floor counts as the last tread" and you get 14 treads.
